Transcriptional cascades are ubiquitous in differentiating systems. They control commitment/cell fate decisions and superimpose order on differentiation programs. Understanding how transcriptional cascades are regulated is important since defective regulation of transcriptional cascades cause a broad spectrum of developmental defects. Understanding how transcriptional cascades are regulated is also critical for understanding differentiation programs at the systems level. Meiotic development in the yeast Saccharomyces cerevisiae (sporulation) is an outstanding model system for studying molecular mechanisms that regulate differentiation. Meiosis-specific genes are repressed during vegetative growth and sequentially induced in three broad groups termed early, middle, and late. The induction of middle genes controls commitment to meiosis, the key irreversible step after which the inducing signal is no longer needed to complete the program. The induction of middle genes is also regulated by a checkpoint pathway that monitors whether preceding events in the program (such as genetic recombination) has been completed and by dependency relationships. Previous studies have shown that the induction of middle promoters is controlled by a transcriptional switch that is controlled by the Sum1 repressor, the Ndt80 activator, and DNA elements in middle promoters termed MSEs. The overarching hypothesis motivating the work in this proposal is that the removal of Sum1 repression is the trigger that permits the expression of NDT80 and meiotic commitment. Completed work has shown that Sum1 recruits a sirtuin (Hst1) to middle promoters, that its nuclear localization is regulated by the small ubiquitin like protein (SUMO), and that the removal of Sum1 repression is regulated by cyclin-dependent kinase and by a meiosis-specific CDK-like kinase named Ime2. The goals of the proposal are: 1- Establish whether SUM1 regulates exit from meiotic prophase through NDT80. 2- Elucidate how Cdk1 and Ime2 co-regulate Sum1 during meiosis. 3. Elucidate how the pachytene checkpoint regulates the middle gene switch. 4. Elucidate how SUMO regulates Sum1 cellular localization during meiosis. These studies will define how commitment to meiotic development is controlled and elucidate molecular mechanisms that control the switch-like properties of this transition. These studies are expected to generate new paradigms for how CDKs and cell-type specific CDK-like kinases collaborate with transcriptional regulators to transiently induce genes in transcriptional cascades. PUBLIC HEALTH RELEVANCE: Transcriptional cascades control virtually all differentiation programs. This project will elucidate how signaling pathways control transcriptional cascades using meiotic development in the yeast Saccharomyces cerevisiae as a model system. These studies will elucidate new molecular mechanisms that regulate meiotic development in particular and differentiation programs in general.